Many applications for vibratory feeders require high conveying feed rates. Feed rates on a vibratory feeder is a function of the operating frequency (number of vibration cycles per second), stroke (displacement magnitude) of the conveying surface, and the stroke angle with respect to a horizontal reference plane.
In a two-mass, electromagnetic vibratory feeder a longitudinal conveying member, the trough, is usually disposed above a base member and connected to the base member by means of a system of springs. The springs are connected to the trough and base members on an angle, such that the spring connection of the trough would be displaced a distance from a vertical reference that passes through the center of the spring connection on the base member. An armature of an electromagnet is connected to one of the base or trough members, usually the trough, and the electromagnet core and coil is connected to the other. The base member is usually isolated from its support structure by coil springs, or elastomer springs to minimize unwanted forces from being transmitted into the support, and surrounding structures.
When an electric current is caused to flow through the magnet, the armature and magnet pole faces are mutually attracted to each other, causing the springs to deflect, and displacing the trough and base from their rest position. When the current is removed, the magnet releases, and the energy stored in the spring system by deflection, causes the trough and base to return to their rest positions to a deflection in an opposite direction to a maximum position where the trough and base will once again change directions toward the rest position. If the current is then reapplied, the process is repeated. If the current is turned on and off at a uniform rate, the trough and base will be reflected at that rate, or frequency.
Typically, electromagnetic driven vibratory feeders are operated at a frequency determined by the power line frequency, or at half of the power line frequency by use of a diode rectifier, or by use of a permanent magnet as part of the electromagnetic drive system. Examples of such feeders are those manufactured by FMC Corporation of Homer City, Pa., under the trade name SYNTRON. In such feeders, the frequency is fixed at 120 Hz or 60 Hz in North America, and 100 Hz or 50 Hz (usually 50 Hz) in most other countries of Europe or Asia. Since the frequency at which these feeders operate is fixed, only the stroke and stroke angle can typically be adjusted to optimize the feed rate. The stroke magnitude of these feeders is constrained by the amount of magnetic force available to deflect the spring system, and ultimately by the stress limitations of the spring system and the structural members of the feeder.
The two-mass feeder takes advantage of the natural amplification of the stroke due to resonance, by adjusting the natural frequency of the mass/spring system to be close to that of the operating frequency. This assures that there will be sufficient power available to operate the feeder with a reasonably sized electromagnetic. Typical maximum stroke values for these feeders operating at 60 Hz. is 0.0625 inches to about 0.1 inches, and at 50 Hz., is about 0.09 inches to about 0.144 inches.
The equation for acceleration of the trough, assuming sinusoidal motion may be stated as:
Eq. #1 a.sub.t =N.sup.2 A.sub.t /70400 PA1 where A.sub.t =the acceleration of the trough along the linear drive line; PA1 N=the operating frequency in cycles per minute; PA1 A.sub.6 =the stroke of the trough in inches; PA1 and 70400 is a constant derived from equation simplification, and conversion to the unit measure value as shown above. PA1 Eq. #2 a.sub.v =a.sub.t sin (.alpha.) PA1 where a.sub.v =the vertical component of the trough acceleration a.sub.t in g's; PA1 .alpha.=the spring angle.
For the trough strokes and frequencies given, the accelerations at both 60 Hz. and 50 Hz. range from 11.5 g's., to 18 g's. As can be seen, the acceleration is heavily influenced by the operating frequency, because the acceleration varies with the square of a change in frequencies, but varies only proportionally to a change in stroke.
As previously stated, feed rate is also dependent on the angle at which the acceleration is applied to the trough member of the feeder. As the trough is linearly accelerated along a path defined by the spring angle, any point on the trough is therefore subjected to both a horizontal and a vertical component of the acceleration. The vertical component, again assuming sinusoidal motion, may be expressed as:
As the trough is accelerated, a particle resting on the trough surface would be accelerated with the trough, and at a point where the vertical acceleration on the particle exceeds -1 g, the particle would separate from the trough's surface, taking flight. The particle, by force of gravity would return to the trough surface displaced at some distance from where it took flight, depending on the amount of acceleration and the angle at which it was applied. Starting with a spring angle of 0.degree., the feed rate would, for practical purposes, also be at 0, as only the horizontal component of the trough acceleration would be present. As the spring angle increases, the feed rate also increases, until the optimum feed rate for that combination of frequency, stroke and spring angle is reached. At some angle, the feed rate would start to decrease again, and will continue to decrease as the spring angle decreases, with violent bouncing of the material being conveyed as the spring angle approaches 90.degree..
If the particles separated from the trough returns to be in contact with the trough within the same vibration cycle, but at a point on the trough acceleration curve where the particle can be again accelerated to the same level as the previous cycle, it is referred to as the "first stable feed zone". Likewise, if the particle leaves the trough, and comes back in contact with the trough in the next vibration cycle, again at a point on the trough acceleration curve where it can stabilize, it would be referred to as conveying in the "second stable feed zone", and so on for particles landing in three or four vibration cycles. There exists between these stable feed zones acceleration regions where the particle cannot be uniformly accelerated between landings. In these unstable zones, feed rate is indeterminate by calculation, but the net result in practice is a decrease from the feed rate obtained just prior to the unstable zone.
While very high feed rates can be realized from operation in the higher stable feed zones, there are practical reasons that make such operation difficult. These regions are very sensitive, because small changes in feed angle, stroke, surface friction, etc., can result in major changes to the feed rate, for example. Also, at the acceleration levels involved, it would be difficult to design structures and spring systems for such high frequency electromagnetic equipment, and have the structures and spring systems survive the stress levels involved. For these reasons, electromagnetic feeders are usually limited to operation in the first or second stable feed zones.
Another concern of operating a feeder in higher acceleration regions is the possible damage to the material being conveyed from high impact speed between the material particles and the trough surface, as material particles, accelerated by gravity, land back on the trough after flight. As an accelerated particle separates from the trough surface in flight, depending on its acceleration and hence its flight time, it may land back on the trough surface such that velocity of the trough adds to the velocity of the particle when they collide, resulting in a high impact force. It is necessary, therefore, to choose combinations of frequency, stroke and feed angle that minimizes impact speed without sacrificing too much feed rate. In the case where the frequency is fixed and the stroke is controllable, it is important to select a feed angle that results in the best compromise between impact speed and feed rate. Often concerns about product damage from high impact collisions between the material and the trough preclude operation in the second and higher feed zones.
In order to have uniform rate along the entire length of the trough member of the feeder, it is advantageous that the feed angle and stroke be uniform along the entire length of the trough member, as well. In order to accomplish this, it has been suggested that the drive force could be applied along a linear, angular path, such that it passes through the center of gravities of the base mass, trough mass, and the effective center of gravity of the system as a whole. By so aligning the drive force, in theory, the trough and base masses would not generate inertial forces about their respective mass centers to form a force couple that would cause the feeder to rotate or to pitch longitudinally on its isolation system.
In practice, it is difficult to align the center of gravities because of the constraints imposed by the geometry of the feeder and its installation constraints. Balancing weights would be required to mount onto the base or trough, as the case may be, to align the center of gravities. The balancing weights add unwanted mass to the feeder, add to cost, and detract from its performance. Often, as in the case of small feeders used to feed product to vertical weigh scales, it would be virtually impossible to dynamically balance the feeder, because the space limitations of the scale require a feeder with an extremely short base.
Prior patents such as U.S. Pat. Nos. 3,216,556, 4,260,052 and 4,356,911 describe methods and apparatus to compensate for the dynamic inertial force couple. These devices compensate by independently adjusting the feeder spring angles relative to one another, such that they cause a rotation of the trough that opposes the rotation caused by the inertial force couple. With this method, however, it is difficult to adjust the spring angles correctly to achieve uniform motion, and each individual feeder manufacturer requires its own unique adjustment. This requirement for adjustment adds considerable cost to manufacturing, making it difficult to compete in a high volume, low cost market. Also, the adjustable spring angle feature places geometric restraints on the feeder design, such as a minimum length requirement, making it difficult to be adapted for use in a weigh scale feeder. For a weigh scale feeder it is important to be able to reduce the size of the feeder, advantageous because such would enable the scale manufacturer to reduce the height and overall size of the equipment, making such equipment more cost effective.
Another problem often encountered in applying electromagnetic feeders, particularly where a high stroke feeder is required to start and stop frequently, is that material continues to feed for an instant after the electric power is removed from the feeder magnet. This phenomena is known as "coasting", and is caused by the combination of inertia and low spring system damping, and can be a problem for the user, particularly in a weighing application, where the overfeed can add up in lost material and lost revenue. External damping means can be added to the feeder to reduce the coasting problem, such as adding dash pots and the like, but since dampers use energy, there often is insufficient power to maintain the high stroke required for the desired feed rate while in operation.
Also, another source of overfeeding after shutoff is that due to the angle of repose of the material, if the material is poured from a container onto a surface unconstrained, it will form a conical pile. The angle formed between the base of the pile and its slope, is the angle of repose for that material. If the bed depth of the feed is very deep, as might be required with lower feed rate feeders in order to maintain capacity, and the feeder is stopped, material falls from the discharge lip of the trough until the angle of repose of the material has been reached. Often in the past, mechanical gates have been used to prevent material from discharging due to this phenomenon, but again this adds to increased equipment costs.
Another concern of users of these feeders include the requirement in some applications of a sanitary service type of construction, for example for food as feed material, with few if any places allowable where particles of feed material can collect. Other concerns include: the ease of cleaning the feeder, an effective vibration isolation of the feeder that minimizes any forces being transmitted to the feeder support structure, a low maintenance with easy access to adjustment devices, and a low operating noise level.
It would be desirable to provide a vibratory feeder that advantageously addresses the above concerns and would be useful in meeting the requirements of the vertical and linear weigh scale feeder markets.
These and other objects, features, and advantages of this invention are evident from the following description of a preferred embodiment of this invention, with reference to the accompanying drawings.